An electronic system can be powered by multiple power supplies, such as a primary power source and a secondary power source. Usually, there is a power management device managing the operation of these multiple power sources. Conventionally, the primary power source may be a power source such as a fuel cell (or an AC/DC adapter) having a limited output power capacity. The secondary power source may be a rechargeable battery pack such as a lithium-ion battery pack. Generally, power supply to the system can be provided by the fuel cell and backed up by the lithium-ion battery pack. If the load of the system is heavy, power demand of the system may exceed the output capacity of the fuel cell. To provide enough power to a system with a heavy load, the power management device may instruct the fuel cell and the lithium-ion battery pack to discharge simultaneously.
The above-mentioned power management device has several drawbacks. The power demand of a system with heavy load may exceed the output power capacity of the fuel cell frequently. Accordingly, the lithium-ion battery pack discharges frequently so as to provide enough power to the system. Frequent discharging of a lithium-ion battery pack may shorten the lithium-ion battery life. Moreover, as the power demand of the system increases, the output power delivered by the lithium-ion battery increases as well. Since the lithium-ion battery pack usually has a limited battery capacity, the energy stored in the lithium-ion battery pack may be exhausted quickly to power a relatively heavy load, which can affect the performance of the system.
To prevent the lithium-ion battery pack from discharging, a primary power source which has a large output power capacity can be used in a conventional power management device. However, the introduction of a primary power source having a larger output power capacity may not only result in more heat dissipation but also increase the system cost.